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Precambrian Research 226 (2013) 143– 156

Contents lists available at SciVerse ScienceDirect

Precambrian Research

journa l h omepa g e: www.elsev ier .com/ locate /precamres

ectonic model of the Limpopo belt: Constraints from magnetotelluric data

. Khozaa,b,∗, A.G. Jonesa, M.R. Mullera, R.L. Evansc, S.J. Webbb, M. Miensopustd, the SAMTEX teamDublin Institute for Advanced Studies, 5 Merrion Square, Dublin, IrelandUniversity of the Witwatersrand, 1 Jan Smuts Avenue, Johannesburg, South AfricaDepartment of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543-1050, USAInstitut fur Geophysik, Westfalische Wilhelms Universitat, Munster, Germany

r t i c l e i n f o

rticle history:eceived 25 June 2012eceived in revised form4 November 2012ccepted 26 November 2012vailable online xxx

a b s t r a c t

Despite many years of work, a convincing evolutionary model for the Limpopo belt and its geometri-cal relation to the surrounding cratons is still elusive. This is partly due to the complex nature of thecrust and upper mantle structure, the significance of anatectic events and multiple high-grade meta-morphic overprints. We use deep probing magnetotelluric data acquired along three profiles crossingthe Kaapvaal craton and the Limpopo belt to investigate the crust and upper mantle lithospheric struc-ture between these two tectonic blocks. The 20–30 km wide composite Sunnyside-Palala-Tshipise-Shear

eywords:impopo beltrchaeanagnetotelluric

Zone is imaged in depth for the first time as a sub-vertical conductive structure that marks a fundamentaltectonic divide interpreted here to represent a collisional suture between the Kaapvaal and Zimbabwecratons. The upper crust in the Kaapvaal craton and the South Marginal Zone comprises resistive granit-oids and granite-greenstone lithologies. Integrating the magnetotelluric, seismic and metamorphic data,we propose a new tectonic model that involves the collision of the Kaapvaal and Zimbabwe cratons ca.2.6 Ga, resulting in high-grade granulite Limpopo lithologies. This evolutionary path does not require a

r eac

separate terrane status fo

. Introduction

Precambrian regions hold the key to understanding the tec-onic processes that prevailed during the early and middlerchaean. Many questions still remain to be answered regarding

he amount of heat available at the time, the onset and domi-ance of early plate tectonic processes and crustal generation theserocesses and/or by plumes. The greatest impediment is the rel-tive paucity of preserved Archaean rocks, compared to inferredrustal generation during the Archaean, and the identification ofrecambrian structures is often masked by secondary tectonicvents.

The highly complex Limpopo belt in Southern Africa provides

natural laboratory to investigate these questions and elucidatehe possible geological processes taking place, where structural,

etamorphic and geochemical data are available. The Limpopoelt is an Archaean-aged high-grade metamorphic complex located

∗ Corresponding author at: Dublin Institute for Advanced Studies, 5 Merrionquare, Dublin, Ireland. Tel.: +27 (0) 71 623 9168.

E-mail addresses: davidkhoza@cp.dias.ie, david.khoza2@gmail.com (D. Khoza).1 Members of the SAMTEX team include: L. Collins, C. Hogg, C. Horan, G. Wallace,.D. Chave (WHOI), J. Cole, P. Cole, R. Stettler (CGS), T. Ngwisanyi, G. Tshoso (GSB),. Hutchins, T. Katjiuongua (GSN), E. Cunion, A. Mountford, T. Aravanis (RTME), W.ettit (BHPB), H. Jelsma (De Beers), P.-E. Share (CSIR), and J. Wasborg (ABB).

301-9268/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.precamres.2012.11.016

h of the Limpopo zones, as has been previously suggested.© 2012 Elsevier B.V. All rights reserved.

between the Kaapvaal and Zimbabwe cratons (Fig. 1). It has anENE–WSW trend and comprises three zones, the Northern andSouthern Marginal Zones and the Central Zone, separated from eachother by major thrust faults or strike-slip shear zones. Metasedi-ments, granitoids and gneisses comprise a significant componentof the rock outcrop. Due to its Archaean affinity and relatively goodsurface exposure, the Limpopo belt has been a focus of a number ofgeological, structural, metamorphic and geophysical studies (VanReenen et al., 1987; Roering et al., 1992; De Beer and Stettler, 1992;Rollinson, 1993; Durrheim et al., 1992). More recently, a Geologi-cal Society of America Memoir (207) on the origin and evolution ofhigh-grade Precambrian gneiss terranes focused specifically on theLimpopo belt (van Reenen et al., 2011).

Several models have been suggested regarding the formationand deformation of the Limpopo belt and these are presentedand discussed in Section 3. Seismic tomography models suggestthat the lithospheric mantle beneath the Limpopo belt is gener-ally similar to that of the Kaapvaal and Zimbabwe cratons, i.e., fastvelocities, implying thick, cold lithosphere (Li, 2011; James et al.,2001). In contrast the crustal structure of the Limpopo belt is highlycomplex, with evidence of polytectonic events that include meta-

morphism, magmatism, crustal uplift and structural deformation(Kramers et al., 2011). The nature of the horizontal movements dur-ing continental accretion, including the orientation and rate of platemovements during the Archaean, is not fully described. In addition,the deep geometry of the margin between the Kaapvaal craton and

144 D. Khoza et al. / Precambrian Research 226 (2013) 143– 156

F therng rofilec ted, sh

ti

prowtdb(ett2MpmmbeUcgpt2AEpKtc

ig. 1. SAMTEX stations (black circles) overlain on the regional tectonic map of Soureen filled circles show locations of the MT stations for the LOW, KAP and LIM-SSO pourtesy of Anamte and Eakins (2009). The blue rectangle shows the area of interes

he Limpopo belt, including the shear zones separating each zone,s not fully known.

In this article we attempt to address several of these issues,articularly the nature of the geometry of the shear zones sepa-ating the cratonic units, and investigate the exotic terrane statusf the Limpopo belt, particularly for the Central Zone. To this ende use magnetotelluric (MT) data to image the crust and the man-

le lithosphere beneath the Limpopo belt and Kaapvaal craton. Theeep-probing MT technique has been used successfully in Precam-rian regions all over the world to elucidate their tectonic historyHeinson, 1999; Davis et al., 2003; Selway et al., 2006, 2009; Sprattt al., 2009). In Southern Africa, the MT method has been usedo image Archaean and Proterozoic boundaries and to understandhe tectonic history of Archaean lithosphere (Jones et al., 2005,009; Hamilton et al., 2006; Muller et al., 2009; Evans et al., 2011;iensopust et al., 2011; Khoza et al., 2011). The primary physical

arameter being investigated is electrical resistivity of sub-surfaceaterials. Given its sensitivity to resistivity contrasts, the crustalodels derived from MT data are very informative in mapping

asement features, the location of deep seated fault blocks (Selwayt al., 2006; Spratt et al., 2009) and crustal melts (Wei et al., 2001;nsworth et al., 2004, 2005; Le Pape et al., 2012). In the Earth’srust the primary conducting mineral phases are saline waters,raphite, sulphides, iron oxides and, in active regions like Tibet,artial melt. Mantle electrical resistivity is primarily sensitive toemperature variation and water content (Jones et al., 2012; Evans,012) and, to a lesser extent chemical composition and pressure.s part of the highly successful Southern African Magneto Telluric

Xperiment (SAMTEX) we have collected MT data along severalrofiles crossing the Limpopo belt and its bounding terranes: theaapvaal craton to the south, Zimbabwe craton to the north and

he Magondi belt to the west (Fig. 1). The LOW profile (yellow filledircles, Fig. 1) crosses the northern Kaapvaal craton, over the Hout

Africa, modified from Begg et al. (2009) and Webb (2009). The yellow, purple ands respectively that are the focus of this work. The bathymetry data are from ETOPO1own in Fig. 2.

River Shear Zone, which is thought to represent the southern limitof the Limpopo belt, into the Central Zone.

The KAP profile (pink filled circles, Fig. 1), part of which wasthe focus of the two-dimensional Kaapvaal craton study of Evanset al. (2011), is re-modelled here using newly developed three-dimensional (3D) techniques and also to do a holistic focused studyof the Limpopo–Kaapvaal boundary. The LIM-SSO profile (green-filled circles, Fig. 1) crosses the Kaapvaal craton, Bushveld complex,Limpopo belt, Magondi belt and/or southern Zimbabwe craton andterminates close to Orapa kimberlite field. The Martin’s Drift kim-berlite cluster (Fig. 2) is about 50 km from MT site SSO103 (Fig. 2).The Orapa and Martin’s Drift kimberlites erupted about 93 Ma and1350 Ma respectively (Haggerty et al., 1983; Jelsma et al., 2004).These profiles, crossing the Limpopo belt and its surrounding ter-ranes, were picked to provide a spatially adequate database toperform 3D magnetotelluric inversion and to understand the natureof the crustal and upper mantle geometry of the geology along theLimpopo belt.

2. Geological background

We define in Table 1 some acronyms that will be referred toconsistently in the text.

The Limpopo belt is an ENE–ESW trending high grade Archaeanmetamorphic complex situated between the lower-metamorphicgrade granite-greenstone Zimbabwe and Kaapvaal cratons. Thethree geologically-defined zones that make up the complex, the

Northern Marginal, Central and Southern Marginal zones, are sep-arated from each other by variously dipping shear zones (Fig. 2).

The Northern Marginal Zone (NMZ), which comprises granite-greenstone material (magmatic enderbites), is separated from theZimbabwe craton to the north by the southward-dipping North

D. Khoza et al. / Precambrian Research 226 (2013) 143– 156 145

F or struK ots inV ginal

LngK

pra(cTf1twSZ

TD

ig. 2. Geology of Limpopo belt (modified from Kramers et al., 2011) showing majAP (purple-filled circles) and LIM-SSO (green-filled circles) are shown. The red denetia kimberlite fields. CZ = Central Zone; KC = Kaapvaal craton; NMZ = North Mar

impopo Thrust Zone (NLTZ). The southern limit of the NMZ (theorthern limit of the CZ) is marked by the south-dipping Trian-le Shear Zone (TSZ) (Roering et al., 1992; Kamber et al., 1995a;ramers et al., 2011).

The Central Zone (CZ) is a 3.3–2.5 Ga high grade zone that com-rises metasediments, S-type granitoid gneisses and supracrustalocks. Two periods of high metamorphic grade metamorphismre recorded in the CZ between 2.7–2.6 Ga and the other at 2.0 GaSmit et al., 2011). The 530 Ma diamondiferous Venetia kimberliteluster is located within the CZ of the Limpopo belt (see Fig. 2).he Palala-Tshipise Straightening Zone (PTSZ) separates the CZrom the SMZ in NE South Africa (McCourt and Vearncombe,992) and has an ENE to NE trend. However, in SE Botswana

he southern margin of the CZ is marked by a composite 40 kmide NW–SE striking linear structure, made up of gneisses of the

unnyside Shear Zone and mylonites of the Palala-Tshipise Shearone. This complex composite structure is thus referred to as the

able 1escription of acronyms referred to in the article.

Acronym Description

KC Kaapvaal cratonSMZ South Marginal ZoneCZ Central ZoneNMZ North Marginal ZoneZC Zimbabwe cratonTSZ Triangle Shear ZonePTSZ Palala-Tshipise Shear ZoneSPTSS Sunnyside-Palala-Tshipise Shear SystemNLTZ North Limpopo Thrust ZoneHRSZ Hout-River Shear ZoneSSZ Sunnyside Shear Zone

ctural and geologic features. The MT station locations of the LOW (yellow circles),dicate the location of known kimberlites, including the Orapa, Martin’s Drift andZone.

Sunnyside-Palala-Tshipise Shear System (SPTSS) and is inferredfrom mineral lineation studies to have sub-vertical dip (Horrocks,1983; McCourt and Vearncombe, 1992).

The Southern Marginal Zone (SMZ) is a 60 km zone that con-sists chiefly of enderbitic and charnokitic gneisses and, unlike theCZ and NMZ, the SMZ experience a single metamorphic eventat 2.72–2.65 Ga only. The north-dipping Hout-River Shear Zone(HRSZ) is thought to mark the boundary between the SMZ and theKaapvaal craton to the south. The 1.9 Ga Soutpansberg basin, whichforms a 40 km wide, 300 km long, 7 km thick volcano-sedimentarytrough, partially cross-cuts the PTSZ and developed as a graben-like basin (Tankard et al., 1982; Kamber et al., 1995a,b; Schalleret al., 1999). Granitoid gneisses and NE-trending greenstone belts(i.e., Murchison, Pietersburg and Giyani belts) make up the geolog-ical composition of the north-eastern Kaapvaal craton (Rollinson,1993). The SMZ and the Kaapvaal craton show uniform and low207Pb/204Pb and206Pb/204Pb isotope ratios in metapelites and leu-cocratic granitoids (Kreissig et al., 2000) suggesting that the SMZ isa high-grade equivalent of the Kaapvaal craton. This result is in con-trast to a separate terrane model for the SMZ proposed by Rollinson(1993), who argued that the different crustal histories and the sep-aration of the SMZ, CZ and NMZ by major thrust faults pointed toseparate Limpopo belt terranes. Similarly, the Zimbabwe craton,the NMZ and the CZ show elevated 207Pb/204Pb and 206Pb/204Pb iso-tope ratios, implying that these three terranes cannot be regardedas separate units from each other and were derived from poten-tially the same source, either in the mantle or crust, having high

U/Pb ratios (Barton et al., 2006; Andreoli et al., 2011; Kramers et al.,2001, 2011).

The geometry of the shear zones bounding and separating themarginal zones and central zones from the cratonic blocks havebeen central to some of the proposed collisional models of the

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46 D. Khoza et al. / Precambria

volution of the Central belt (Durrheim et al., 1992; De Beer andtettler, 1992; Van Reenen et al., 1987; Roering et al., 1992; Treloart al., 1992). The Kaapvaal craton forms the core of the compositealahari craton and formed in Archaean times ca. 3.7–2.6 Ga (de Witt al., 1992). Post-stabilization processes include the developmentf sedimentary basins (e.g., Witwatersrand basin) at ca. 3 Ga andere followed by extensive volcanism (Ventersdorp magmatism)

Eglington, 2004). The very widespread platform sedimentationf the Transvaal Supergroup that overlies the Ventersdorp Super-roup represents exceptional early stability of the Kaapvall craton.ushveld complex (Fig. 2), which intruded into the Kaapvaal cratonnd1 upper-most Transvaal sequence, is the largest known layeredntrusion on Earth and its emplacement significantly altered thehermal and chemical structure of the Kaapvaal craton at 2.06 GaJames et al., 2001; Evans et al., 2011). The Zimbabwe craton, whichs also part of the greater Kalahari craton, is itself thought to be com-osed of a number of distinct tectonostratigraphic terranes, whichonsist of 3.5–2.95 Ga gneissic rocks overlain by 2.92 Ga assemblagef mafic and felsic volcanic rocks at its core (Blenkinsop and Vinyu,997; Kusky, 1998). Several 3.5–2.6 Ga greenstones belts completehe lithological profile of the Zimbabwe craton (Blenkinsop andinyu, 1997).

. Tectonic models of Limpopo belt

The complex nature of the geological and structural relation-hips in the Limpopo belt has led to several plate tectonic andon-plate tectonic models being proposed for its evolution. Theseere detailed and reviewed by Kramers et al. (2011) and are sum-arized here.

.1. The Neoarchaean Himalayan model

Treloar et al. (1992) were the first to propose a model of theimpopo belt involving continental growth by accretion followedy shortening, which is similar to the Mesozoic evolution of Tibetanlateau during India–Asia collision. Central to this model was thebservation of a regional structural pattern that suggest NW–SEompression, resulting in crustal thickening that also involved fold-ng and NW-directed thrusting and lateral extrusion of crustallocks along SW- to WSW-trending shear zones. Treloar et al.1992) thus argued that terrane status (i.e., that each unit formedeparately as unique/discrete crustal or lithospheric blocks prior tomalgamation and accretion as the Limpopo belt) for the SMZ, CZnd NMZ was not required, and that the Limpopo belt was a resultf a crustal deformation event that included much of the Kaapvaalnd Zimbabwe cratons. Roering et al. (1992) argued that the gran-lite terrane was a result of crustal thickening in response to theorthward thrusting of the Kaapvaal craton over the Zimbabwe cra-on along the Triangle Shear Zone, ca. 2.7–2.6 Ga. This was followedy a metamorphic event and subsequently isothermal decompres-ion during which rocks moved upward and spread outward ontohe adjacent cratons, during a period of widespread anatexis, cre-ting what has since being called a pop-up structure (Roering et al.,992).

.2. Terrane accretion models

Rollinson (1993) and Barton et al. (2006) proposed modelsescribing the Limpopo belt formed by accretion of separate ter-anes of unrelated origin that constitute the NMZ, CZ and SMZ.

n Rollinson (1993)’s model the distinct crustal evolutions of theones, supported by prominent shear zones separating them,mplied these blocks accreted together prior to the collision ofetween the Kaapvaal and Zimbabwe cratons in Neoarchaean andarrant their consideration as discrete terranes.

arch 226 (2013) 143– 156

Barton et al. (2006) proposed a similar accretion model, but,unlike Rollinson (1993), the process involved a complex assem-bly of a large number of terranes between ca. 2.7 and ca. 2.04 Ga,where the SMZ, CZ, NMZ, Zimbabwe craton, Phikwe and Beit Bridgecomplexes, accreted, in subduction settings, to form migratingarcs that led to development of juvenile crust. This Turkic-typeaccretion was proposed by Sengor and Natal’in (1996) as theprincipal craton building process through Earth’s history. In thismodel the Beit bridge and Phikwe complexes, in addition tothe terranes defined by Rollinson (1993), show distinct P–T–t(pressure–temperature–time) paths and metallogenic signatures,that suggest they are separate terranes. Furthermore, the lackof S-type granitoid magmatism, ophiolites and syntectonic sed-imentary basins led Barton et al. (2006) to argue against thecontinent–continent collision model (Kramers et al., 2011).

3.3. Transpression model for the central zone

In contrast to McCourt and Vearncombe (1992), who argued fora dip- or oblique-slip movement along shear zones (see below),Kamber et al. (1995a,b) interpreted the movement along the TSZand PTSZ as being dextral-transcurrent that recorded the Paleopro-terozoic transpressive collisional event which resulted in crustalthickening and uplift of the CZ. This model was later expanded byHolzer (1998) and Schaller et al. (1999).

3.4. Other models

Other models that have been invoked include that of McCourtand Vearncombe (1992) who, based on Pb isotopic and struc-tural grounds, interpreted the CZ to be an exotic terrane thatwas emplaced from NE to SW as a thrust sheet facilitated by theTriangle-Tuli-Sabi and Sunnyside-Palala structures which acted ascomplimentary lateral ramps. Based on Pb isotope data on igneousrocks and the dip slip shear movement along these structures,which resulted in the CZ being the structurally the highest in rela-tion to the marginal zones, McCourt and Vearncombe (1992) arguedthat the CZ may have represented a part of the overriding plateduring the Neoarchaean Limpopo orogeny (Kramers et al., 2011).

4. Previous geophysical studies of the Limpopo belt

Several studies have been performed, each attempting to imagethe geometry and structure of the Limpopo belt. These are summa-rized in Fig. 3.

The geoelectric, seismic reflection and gravity results of DeBeer and Stettler (1992) and Durrheim et al. (1992) formed thebasis from which many of the collisional models were formulated(Fig. 3D). The northward and southward dip of HRSZ and the NLTZrespectively lend support to the ‘pop-up’ model derived by Roeringet al. (1992), as outlined above. Furthermore, the gravity resultsdelineated high density rocks in the upper crust of the NMZ andSMZ. The PTSZ did not correspond with any seismic reflection signa-ture which led De Beer and Stettler (1992) to propose that it is a nearvertical fault that penetrates the entire crust. The work of Jameset al. (2001) used P-wave and S-wave delay times from a broad-band seismic array to map high velocity mantle roots extendingto depths of 250–300 km beneath the Kaapvaal and Zimbabwe cra-tons and the Limpopo belt (Fig. 3B). In a related crustal study, Nguuriet al. (2009) analyzed receiver functions to map the crustal struc-

ture of the Limpopo belt (Fig. 3A). The NMZ was found to have a37 km thick crust, similar to the Zimbabwe craton, while the SMZhad a 40 km thick crust similar to Kaapvaal craton and the Mohoin both zones generated strong P-to-S conversions. However, theMoho structure from Nguuri et al. (2009)’s study appeared more

D. Khoza et al. / Precambrian Research 226 (2013) 143– 156 147

F of NguL graphD a to stm l struc

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ig. 3. Summary of geophysical studies undertaken over the Limpopo belt. The studyimpopo and Zimbabwe craton using seismic receiver functions (A), while the tomoe Beer and Stettler (1992) and Durrheim et al. (1992) used reflection seismic datore recently Gore et al. (2010) used seismic receiver functions to study the crusta

omplex beneath the CZ, corresponding with weaker P-to-S con-ersions.

In a second attempt, using additional seismic stations, Gore et al.2010) analyzed receiver function data and derived a Moho mapFig. 3C) beneath the entire Limpopo belt which appears to showhick (more than 50 km) crust for the Kaapvaal and Zimbabweratons, which are consistent with estimates from Nguuri et al.2009)’s results. As noted by Gore et al. (2010), the Limpopo belt has

low elevation relative to the adjacent cratons, which is puzzlingn isostacy grounds given the deeper Moho. The authors explainhis discrepancy by interpreting the CZ as a remnant of a deep-ooted crustal block that did not fully rebound during denudation.he notion of a dense lower-crustal/upper-mantle root beneath theZ is supported by the positive Bouger anomaly and could possiblye the result of magmatic underplating (Gore et al., 2010; Gwavavat al., 1992; Ranganai et al., 2002; Kramers et al., 2011).

. The magnetotelluric (MT) method and data

The magnetotelluric method is an electromagnetic (EM) sound-ng technique that has evolved rapidly since its first theoretical

escription in the 1950s. By measuring the time variations on theurface, of the horizontal electric (Ex, Ey) and horizontal and verti-al magnetic (Hx, Hy and Hz) fields induced in the subsurface, wean derive the lateral and vertical subsurface variations of electricalesistivity. The ratios of the EM fields are related, in the frequency

uri et al. (2009) focussed on defining the crustal structure across the Kaapvaalcraton,y work of James et al. (2001) focused on the deep mantle lithospheric structure (B).udy the crustal shear zones bounding the marginal zones of Limpopo belt (D) andture of the Limpopo Central Zone.

(ω) domain, by an impedance Zxy(ω) = Ex(ω) = Hy(ω), from whichthe apparent resistivity (i.e., the resistivity of a homogeneous halfspace)

�a,xy(ω) = 1ω�

∣∣∣∣

Ex(ω)Hy(ω)

∣∣∣∣

(1)

and impedance phase

�xy(ω) = arctan(Ex(ω))Hy(ω)

(2)

can be estimated (Chave and Jones, 2012).In Southern Africa, we have acquired broadband (periods from

0.001 s to 8000 s) and long period (15 s to over 10,000 s) magne-totelluric data as part of the SAMTEX project (Fig. 1). More than750 stations of data were collected in Namibia, Botswana and SouthAfrica over four field seasons along various profiles from 2003 to2008, with station spacings of approximately 20 km and 60 km forbroadband and long period data, respectively. The orientations ofthe profiles were chosen to transect over specific geological fea-tures of interest. Magnetotelluric broadband data were collectedusing Phoenix Geophysics (Toronto) MTU5 instruments, and long

period data were acquired with LVIV (Ukraine) LEMI systems. Inthis work we model MT data collected along three profiles: LOW,LIM-SSO and KAP (Fig. 2).

The NE–SW KAP line was the focus of the Kaapvaal lithosphericstudy of Evans et al. (2011). We focus here specifically on the

148 D. Khoza et al. / Precambrian Research 226 (2013) 143– 156

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ig. 4. MT responses curves for 4 sites, showing apparent resistivity and phase plotteTE) mode data, and the blue squares show the transverse-magnetic (TM) mode da

orthern half of the profile traversing over the Limpopo belt. Thereere several motivations for remodelling this part of the profile.

irstly, the work of Evans et al. (2011) focussed principally onefining the Kaapvaal craton lithosphere not the Limpopo belt.econdly Evans et al. (2011) developed lithospheric models usingD techniques. While 2D modelling is able produce first-orderegional features, it cannot account for 3D data complexities,articularly off-profile features. To address this deficiency wepply a newly-developed 3D inversion algorithm (Egbert andelbert, 2012) to derive information on the 3D structure of theimpopo belt. To this end only 14 stations from the KAP profileere modelled, crossing the northern limb of the Bushveld Igneousomplex (BIC), the SMZ and the CZ (purple sites in Fig. 2).

The almost NNW-SSE LOW profile comprises 12 stations cross-ng the northern Kaapvaal craton in the south, extending into theMZ and part of the CZ (yellow sites in Fig. 2). Vertical magneticeld (Hz) data were recorded at 3 stations only on the LOW profile.ue to logistical and security concerns of going into Zimbabwe at

nst increasing period (proxy for depth). The red squares show the transverse-electric

the time of the survey, the LOW profile was terminated close to theSouth Africa–Zimbabwe border.

The NW–SE LIM-SSO profile comprises 25 stations (13 of whichrecorded Hz data) crossing (from SE to NW) the Kaapvaal craton, thenorthern limb of the BIC, the western Limpopo belt and the south-western margin of the Zimbabwe craton (green sites in Fig. 2).

5.1. MT data and processing

The recorded electric and magnetic time series data wereprocessed using standard robust processing methods of Jones andJodicke (1984), Egbert (1997) and Chave and Thompson (2004)(methods 6, 7 and 8 in Jones et al., 1989).

Given that multiple sites were recorded simultaneously, weemployed remote referencing methods (Gamble et al., 1979) toreduce bias effects and improve the quality of the estimated MTresponses. The resulting responses are shown in Fig. 4 for four rep-resentative stations on and off the Limpopo belt, where variation

D. Khoza et al. / Precambrian Research 226 (2013) 143– 156 149

F ent reS he Lim

ibd

KppcSie

ptetrritfrCtS

6

a

ig. 5. Pseudosections of TE and TM mode data for the three profiles, showing apparome features, like the Kaapvaalcraton upper crust and the resistive lithologies of t

n apparent resistivity is plotted as a function of period, the lattereing a proxy for depth (i.e., the longer the period the deeper theepth of penetration).

Data quality was generally very good for most stations on theAP and LIM-SSO profiles. The LOW profile sites suffered from longeriod distortion and at most stations we were only able to modeleriods up to 300 s. However, the general resistive nature of therust, inferred from resistivity studies of De Beer et al. (1991) in theMZ and northern Kaapvaal craton (high and low grade granitoids),mplied that depth of penetration up to 100 km was assured (alsostimated with 1D Niblett-Bostick approximations).

Fig. 5 shows pseudo-section plots of apparent resistivity andhases for the LOW, KAP, LIM-SSO profiles giving an indication ofhe lateral variation of resistivity with period (i.e. depth). The appar-nt resistivity and phase pseudo-sections are shown for both theransverse magnetic (TM) and transverse electric (TE) modes. Highesistivity values are represented by blue (cold) colours, whereased (hot) colours indicate low resistivity values. Although thesemages are distorted due to the representation of distances alonghe abscissa versus log (period) instead of true depth, some majoreatures can already be recognized. The LOW profile in particulareveals the resistive lithologies of the Kaapvaal craton, SMZ andentral Zones of the Limpopo belt. We will discuss key featureshat are resolved from 3D inversion modelling of these data in aection 7.

. 3D inversion modelling

The motivation for doing 3D instead of 2D inversion is that nossumption about the dimensionality of the data had to be made.

sistivity and phases as a function of period increasing downwards (proxy for depth).popo belt Central Zone are readily recognizable.

Furthermore, modelling the four components of the impedancetogether with vertical transfer functions (Hz) enable us to definethe nature of the structures that would otherwise not be resolvedby applying 2D inversion modelling, thus we are able to gain addedinformation about the resistivity distribution at depth.

In total, MT data from a total of 51 stations were modelled. Themodular EM code of Egbert and Kelbert (2012) was used to gen-erate 3D models. The apparent resistivity error floors were set to10% for the diagonal and 15% for the off-diagonal elements of theimpedance tensor (i.e., if the errors were less than 10% or 15% of theamplitude of the impedance, they were set to that level, if they weremore, they were unchanged). The phase error floors were set to 5%and a constant absolute error floor in Hz was set at 0.01%. The sizeof the 3D grid was 79, 72 and 52 cells on the north, east and verticaldownwards direction, respectively. The impedance elements (Zxx,Zxy, Zyy, Zyx) and Hz were modelled with the smoothing parametertau (�) set to 3 and using a 100 ohm-m half-space as an input model.The final model produced, converged to an average RMS of 3.61(Fig. 9). In order to validate the features observed in the 3D modeland test the resolution of nearby off-profile conductors, we con-ducted 3D inversion of each of the 2D profiles separately (3D/2D).Siripunvaraporn et al. (2005) demonstrated the advantages of mod-elling data this way using synthetic examples and, in a more recentstudy, Patro and Egbert (2011) applied similar technique to modeldata from the Deccan Volcanic Province. The principal advantage is

that off-profile features are correctly located spatially, and are notartificially placed beneath the profile, as can be the case in 2D. Inorder to maintain consistency we use the same inversion parame-ters (i.e., four impedance elements Zxx, Zxy, Zyy, Zyx were modelled,using a tau value of 3 and 100 ohm-m half-spaces as input).

150 D. Khoza et al. / Precambrian Rese

Fig. 6. 3D E-W perspective view showing the variation in resistivity laterally anddbPc

7

dtlt

FKp

epth across the Limpopo belt. The location of the Orapa and Martin’s Drift kim-erlite fields are projected. The dark solid line shows the approximate trace of thealala-Tshipise-Sunnyside Shear system. SPSZ: Sunnyside-Palala Shear Zone. Theonductive Feature A is explained more in the text.

. Results

Our final 3D model (in perspective view) from inverting allata simultaneously is shown in Fig. 6, and horizontal depth sliceshrough the volume are shown in Fig. 7. The blue triangles show theocations of the MT stations and the off-profile extent of the sec-ions is limited to the corresponding depth-footprint. Highlighted

ig. 7. Crustal depth sections derived from 3D inversion model. Three crustal depth are shramers et al., 2011). The main features are highlighted, including the locations of the Orarofile.

arch 226 (2013) 143– 156

on both figures are the SMZ, CZ, PTSZ, HRZ, Bushveld complex andKaapvaal and Zimbabwe cratons. On comparing the 10 km depthslices to the locations of major shear zones, it is clear that the PTSZcorresponds to a major resistivity contrast down to lower crustallevel. In order to obtain an indication of the geometry and extentof the structures at depth, 2D sections were extracted from the 3Dmodel and are shown in Fig. 8. We will now highlight the majorfeatures resolved for each profile.

7.1. LOW profile

The crust in the southern part of the model is dominated byresistive features in the SMZ and the Kaapvaal craton. There isa significant resistivity break, up to 20 km in lateral distance, inthe model at sites LOW003 and LOW004 that correlates spatiallywith the Soutpansberg basin (see Fig. 2). The 1.9 Ga elongatedSoutpansberg Basin occurs within the SMZ on the southern sideof the PTSZ and partly transcends the location of the PTSZ. The3D/2D models suggest that the basin extends off-profile to theeast and west confirming its E–W orientation. The location ofthe PTSZ is characterized by a conductive signature. Based on itslack of seismic response, Durrheim et al. (1992) and De Beer andStettler (1992) interpreted the PTSZ to have a sub-vertical dip.The sites KAP074 and LOW001 are clustered around the 2.57 Ga

Bulai pluton. Compositionally the pluton is charnokitic, madeup of granites and granodiorites and as a result it appears as aresistive feature. Conductive Feature B beneath the SMZ occurs at35 km depths and similar to the enigmatic feature imaged by DeBeer et al. (1991) using DC resistivity and LOTEM methods. Moho

own. The 10 km section is overlain on the geological map of the Limpopo belt (afterpa and Martin’s drift kimberlite fields which are in close proximity to the LIM-SSO

D. Khoza et al. / Precambrian Research 226 (2013) 143– 156 151

Fig. 8. LOW, KAP and LIM-SSO profiles derived from 3D inversion model. The Moho depth on the LOW profile was derived from the seismic receiver function study of Goree projecD ferredS

df

7

ficdapacacmm

t al. (2010) (note the profile depth is 100 km). The location of Venetia kimberlite is

rift kimberlite clusters are shown on the LIM-SSO profile. Features A and B are rehear System.

epth was estimated by Gore et al. (2010) from seismic receiverunctions, and infers a relatively thicker crust beneath the CZ.

.2. KAP profile

Similar features to the LOW profile are observed on the KAP pro-le. The resistive granite-greenstones lithologies of the Kaapvaalraton to the south are mapped in the upper crust. A significant con-uctivity break in crustal structure is observed with a conductivenomaly corresponding to the northern limb of the Bushveld com-lex. The PTSZ is situated along sites KAP068, KAP069 and KAP070nd is similarly evident as a conductive feature. The resistive upper-

rustal Beitbridge Complex lithologies extend to a depth of 10 kmnd are underlain by conductive Feature A. Given the tectonic impli-ations of the location and geometry of the PTSZ, several 3D forwardodels were generated in order to obtain a geometrical model thatatches the observed resistivity responses. To this end, we tested

ted on the KAP profile and is shown as red arrow. Similarly, the Orapa and Martin’s to in the text. PTSZ: Palala-Tshipise Shear Zone, SPTSS: Sunnyside-Palala-Tshipise

models where a 15–20 km conductive zone (approximately 10 �m,akin to PTSZ) is embedded in a resistive media (10,000 �m, akin tothe CZ and Kaapvaal). Various dip angles were tested and the modelwith the conductive zone having a sub-vertical dip returned almostsimilar responses to those of KAP068 and KAP069 (Fig. 8).

7.3. LIM-SSO profile

The SE part of this profile is characterized by resistive low gradelithologies related to the Kaapvaal craton and a conductive fea-ture extending to depths of over 100 km attributed to the northernlimb of the Bushveld Igneous Complex. The prior 2D isotropic and

anisotropic models of Evans et al. (2011) mapped the Bushveldcomplex as a mantle conductive feature extending to depths inexcess of 150 km. The composite SPTSS is positioned on the geo-logical map between MT sites LIM002 and SSO101. This regioncorresponds to a vertically-dipping conductive feature on the MT

152 D. Khoza et al. / Precambrian Rese

Fs

matS4tSadt

8

tSpa(

lwZKBraB1DBF(CdwP(soslg

d

ig. 9. RMS misfit between modelled data and resulting model plotted from eachtation.

odel. The SPTSS is composed of gneisses and mylonites (McCourtnd Vearncombe, 1992), therefore the observed elevated conduc-ivities are interpreted to map the lateral and depth extent of thePTSS. (McCourt and Vearncombe, 1992) mapped the SPTSS as a0 km wide linear shear system that maps the boundary betweenhe CZ and the SMZ. From the resistivity models we infer that thePTSS has approximately 50 km lateral extent and that it represents

fundamental crustal suture zone. The locations of the Martin’srift and Orapa kimberlite clusters are projected on the model andhe lithosphere beneath it is characteristically resistive and thick.

. Interpretation

Several features are resolved from 3D inversion modelling: (1)he upper crustal resistive granite-greenstone of the Kaapvaal andMZ, (2) the lower crustal conductors labelled A and B on the LOWrofile and A on the KAP profile, (3) the conductive feature associ-ted with the composite Sunnyside-Palala-Tshipise Shear SystemSPTSS).

On the LOW and KAP profiles (in South Africa) this shear zone isabelled the Palala-Tshipise Shear Zone (PTSZ) and it extends west-

ard (into NE Botswana) where is links with the Sunnyside Shearone (SSZ) and becomes a composite SPTSS. In the region of sitesAP071 to KAP075 (Fig. 2), the upper crust is composed of Beitridge Complex lithologies, which comprises quartzo-feldspathicocks with a granitic bulk compositions (Klemd et al., 2003). Thesere evident in the LOW and KAP profiles (Fig. 8) as resistive features.elow these rocks, at 10 km depth, is a conductive (approximately0 �m) Feature A. Seismic results (De Beer and Stettler, 1992;urrheim et al., 1992) indicate a reflector at 10 km depth below theeit Bridge complex, corresponding to the top of the conductiveeature A. Conductive middle to lower crust is observed globallyJones et al., 1992; Hyndman et al., 1993) and in the Canadianordillera, for example, coincident reflective and conductive mid-le to lower crust is observed (Marquis et al., 1995), the causes ofhich are thought to be, principally, trapped saline pore-fluids for

hanerozoic regions or graphite for Precambrian regions. Pretorius2003) combined xenolith-derived P–T mineral equilibria data witheismic information to derive the crustal and upper mantle lithol-gy structure beneath the 520 Ma Venetia kimberlite cluster. This

tructure consists of supracrustal rocks down to 10 km depth, over-ying mafic amphibolite rocks and restites (garnet-quartz rocks,ranulite and eclogite).

According to Pretorius (2003), the amphibolites/restites wereerived from partial melting of a remnant subducted Fe–O rich

arch 226 (2013) 143– 156

Archaean oceanic crust and gravitationally settled at deeper levelsdue to their higher densities (3.2–4.5 g/cm3). However it is unlikelythat Feature A corresponds to the amphibolite in that silicate rocksare usually resistive (Chave and Jones, 2012; Evans, 2012); there-fore another conducting mineral phase must be present to accountfor the observed high conductivities. Conductive Feature A extendsfrom 10 km depth to about 50 km, although, given the shieldingeffect of conductive features, the bottom depth is possibly overes-timated. The top of the conductor overlain by a resistor is usuallywellresolved with the MT method. There is a clear spatial corre-lation between the location of Feature A with high density rocks(Ranganai et al., 2002) and the interpreted thickened crust (Goreet al., 2010). In their review of age determinations in the LimpopoComplex, Kramers and Mouri (2011) have commented on the sharpage peak of the 2.0 Ga event in the Central Zone, and they andKramers et al. (2011) have suggested that there may have beenunderplating by Bushveld complex related mafic magmas. Theyargued that this could also explain the gravity anomaly and poorlydefined Moho in the region. Given the known conductive signatureof the Bushveld complex, this could provide an alternative expla-nation of Feature A. It is thus worthwhile to investigate the possiblecauses and tectonic significance of this Feature A.

In the Earth’s crust, fluids, interconnected sulphides/oxides,graphites, or high conducting metamorphic rocks are potentialcandidates for material that can give rise to observed elevatedconductivities. The presence of fluids, particularly in Precambrianterranes, is diffcult to discern due to the complex evolution ofmetamorphic rocks. There is evidence of prograde and retrogrademetamorphism, where rocks have undergone more than one defor-mation event in the Limpopo belt. For this reason, Barnes andSawyer (1980) argued that it is unlikely that fluids will haveremained stable in the crust since Archaean times, given their shortresidence times. Also argued by Yardley and Valley (1997), withinteresting discussion by Wannamaker (2000) and response byYardley and Valley (2000). Goldfarb et al. (1991) gives evidencefor at least 70 Ma residence times of water.

Fluid inclusion studies in the Limpopo belt have focused inthe Central Zone (Hisada and Miyano, 1996; Hisada et al., 2005;Tsunogae and van Reenen, 2007; Huizenga et al., 2011) and theSouth Marginal Zone (van Reenen and Hollister, 1988; van Reenenet al., 1994; van den Berg and Huizenga, 2001; Touret and Huizenga,2001). In the SMZ, fluid inclusion studies (and high-temperaturereaction texture) in granulites (Touret and Huizenga, 2001; van denBerg and Huizenga, 2001) indicate presence of brines and CO2 rich-fluids during peak granulite facies metamorphism. van den Bergand Huizenga (2001) suggest that the brines represents remnantsof preserved connate water. Studies by Huizenga et al. (2011) inthe Central Zone confirmed the presence of CO2-rich-fluids in high-temperature Mg-rich garnets co-existing with brines, similar to theSMZ.

While presence of fluids in the CZ and SMZ is known, questionscan be asked as to (1) which tectonic process introduced CO2 in thecrust, (2) how widespread are they, and, more importantly for elec-trical conductivity, (3) what is the nature of the inter-connectivityof the fluids? The fluid inclusion studies undertaken on materialfrom the Limpopo belt are unable to estimate absolute fluid con-tent and, as such, questions (2) and (3) are beyond the scope of thisstudy, but we address here the first question and suggest a possiblemantle source for CO2, in a subduction setting.

Sm–Nd and Lu–Hf results suggest that the southern Zimbabwecraton was an active magmatic arc from the south to the south-

west, characterized by subduction of oceanic lithosphere ca.2.7–2.6 Ga (Bagai et al., 2002; Kampunzu et al., 2003; Zhai et al.,2006; Zeh et al., 2009; Kramers et al., 2011; Kramers and Zeh,2011). Furthermore, the CZ, NMZ and Zimbabwe craton have sim-ilar elevated U/Pb ratios and U, Th concentrations (Kramers et al.,

n Rese

2astdfiirzsSmtapsm

tdatsFBtttBsit

MG1Riititlardc

m(selWatrcSantctV

D. Khoza et al. / Precambria

001; Barton et al., 2006; Andreoli et al., 2011), implying thatll three represent parts of the same plate where the CZ is thehelf, the NMZ the active margin and the Zimbabwe craton beinghe overriding plate (Kramers et al., 2011). These results contra-ict the Turkic-type model that requires separate terrane statusor the CZ, NMZ and Zimbabwe craton. The presence of subduct-ng carbonate-rich oceanic lithosphere would release CO2 fluidsnto the lower crust and react with the mantle peridotite of shelfegion (CZ), evidence of the latter process in the CZ stems fromircon Lu–Hf data on the charnockitic Bulai pluton sample whichhows remelting of older crust (Zeh et al., 2007). However, theM-Nd studies of Harris et al. (1987) suggested that the plutonight have been derived from anetexis of juvenile crust. In addi-

ion, petrographic study of 65 samples exposed in a 10 km2 area,pproximately 13 km west of site KAP074 (Fig. 2), indicated theresence of graphite and magnetite (Klemd et al., 2003). In theame study, CO2-rich inclusions were found in garnet rims ofetapelites.It is therefore reasonable that graphite was precipitated from

he CO2-rich fluids. We therefore propose that the observed con-uctivity of Feature A is due to the presence of graphite and minorccessory minerals like magnetite, in the crust. The graphite reduc-ion mechanism was invoked to explain the high conductivityignatures of the Fraser fault and also proposed for Yellowknifeault. The conductive feature B is located in the lower crust. Deeer and Stettler (1992) mapped it, but provided no explanation athe time as to its possible cause and significance. In our models andhose of De Beer et al. (1991) conductive Feature B coincides withhe position of the HRSZ at depth; it is not known if Features A and

is are tectonically related. However given that two independenttudies using different techniques have now confirmed its presencet is reasonable to infer that conductor B is a pervasive feature inhe crust.

Conductivity in the Earth’s lower crust has been observed fromT studies and debated for many decades (Edwards et al., 1981;regori and Lanzerotti, 1982; Jones et al., 1992; Touret and Marquis,994; Glover, 1996; Yang, 2011), but there, is yet, no consensus. vaneenen and Hollister (1988) suggested a subdivision of the SMZ

nto a northern granulite zone and a zone of retrograde hydrationn the south, the former being the result of prograde melting reac-ions without involving fluids (Stevens, 1997) whereas the latters associated with CO2 and brine rich fluids sourced from devoli-isation reactions (van den Berg and Huizenga, 2001). Feature B isocated within this hydrated subzone but it is unlikely that fluidsre responsible for the observed conductivities given their shortesidence times in the crust, therefore graphite (and minor con-uctive phases like FeO and sulphides) are suggested to be likelyandidates for increased conductivities.

Central to the pop-up model proposed for the Limpopo develop-ent was the geophysical mapping studies by De Beer and Stettler

1992) and Durrheim et al. (1992), who imaged the northward andouthward dip geometry of the HRZ and TSZ respectively. The PTSZxhibited no seismic response in the study of Durrheim et al. (1992),eading to the suggestion that it was a vertically-dipping feature.

hile our results do not dispute the previous results of De Beernd Stettler (1992) and Durrheim et al. (1992), we map, for the firstime, the PTSZ as a conductive feature and suggest that it in fact rep-esent a fundamental suture between the Kaapvaal and Zimbabweratons. The PTSZ appears to be sub-vertical on the KAP and LIM-SO profiles and dips slightly to the north on the LOW profile. On

mantle lithospheric scale, the PTSZ correlates with the disconti-

uity observed between the 300 km thick Kaapvaal/SMZ block andhe 250 km thick Zimbabwe/NMZ/CZ block (Fouch, 2004). The PTSZomprises conductive mylonite and ultramylonite rocks, similar tohose found in some parts of the Bushveld complex (McCourt andearncombe, 1992).

arch 226 (2013) 143– 156 153

The conductive region in the Bushveld crust is related to metal-lic sulphides and oxides widespread in the complex (the Bushveldcomplex is the largest resource for platinum group metals). Thelocation of the Northern Limb of the BIC is on a junction between theHout River Shear Zone and the Palala-Tshipise Straightening zone.These shear zones are both characterized by high conductivity sig-nature. Kamber et al. (1995b) noted that high grade metamorphismat 2.0 Ga, which was a result of collision, was coeval with dextraltranscurrent shear movement of the PTSZ, suggesting a transpres-sive collisional event. Given that the Soutpansberg basin is locatedin the SMZ just south of the PTSZ, it is possible that the depositionof volcano-clastic sediments in the trough developed on the exten-sional side of the transpressive shear zone system (post CZ uplift),an observation suggested by Kramers et al. (2011). This model isin contrast with the aulocogen model proposed by Jansen (1975),and is in agreement with a half-graben setting suggested by Bumbyet al. (2002) and Tankard et al. (1982).

The geometry of inter-cratonic sutures have played a significantrole in the evolution of the African tectonic landscape, by focussingascending magmas and areas of localized rifting (Begg et al., 2009).The close spatial proximity of the PTSZ and the Venetia and Martin’sdrift kimberlite suggest that these shear zones could have possiblyacted as conduits to ascending magma, leading to the emplacementof kimberlite volcanic material. The projection of the Orapa kimber-lite on the LIM-SSO MT model suggests that it plots on the part ofresistive thick lithosphere that is an extension of the Zimbabwecraton, as was suggested by Miensopust et al. (2011). The resistivelithosphere in this region on the LIM-SSO profile extends to 150 kmdepth. Seismic tomography maps, however, infer a seismically slowmantle beneath the Orapa kimberlite field (James et al., 2001) whichwas attributed to intrusion events related to mid-Proterozoic colli-sion of the Okwa and Magondi belts (Shirey et al., 2002; Griffin et al.,2003) that significantly modified the-then Archaean lithosphere.

9. Towards a tectonic model

From the discussions above and combining the resistivity mod-els and the recent metamorphic results, we propose a model for theevolution of the Limpopo belt (illustrated in Fig. 10) that involvesthree main tectonic processes, namely (1) subduction phase at2.7–2.6 Ga, (2) collision phase at 2.6–2.5 Ga and (3) transpressionphase at 2.2–1.9 Ga. In this model, the Neoarchaean collision of theKaapvaal and Zimbabwe cratons, preceded by oceanic lithosphericsubduction beneath the latter, is followed by Paleoproterozoic tran-scurrent/transpression along shear zones. The SPTSS representsa major suture zone between the Kaapvaal and Zimbabwe cra-tons.

The Subduction phase (Fig. 10A): petrological results suggest thatthe southern Zimbabwe craton, in effect the NMZ, was an activemagmatic arc characterized by subduction of oceanic lithosphereca. 2.7–2.6 Ga (Bagai et al., 2002; Kampunzu et al., 2003; Zeh et al.,2009). Magmatism was during convergence but prior to collision ofthe Kaapvaal craton with the Zimbabwe craton. At 2.7 Ga, howeverthere is no evidence that the SMZ was an accretionary margin, butthere is an indication that high grade metamorphism was prevalent(Kramers et al., 2011).

The Collision phase (Fig. 10B): the ensuing collision between theKaapvaal and Zimbabwe cratons gave rise to the observed high-grade metamorphism in the CZ and resulted in thickened crust.Shallow syntectonic melting resulted in the intrusion of the gran-

odiorites, such as the 2.6 Ga Bulai pluton, which shows signaturesof old and juvenile crustal anetexis at this time. Deeper melting ofremnant oceanic crust resulted in CO2-rich fluids migrating intothe crust and precipitating graphite. The collisional suture zonebetween the Kaapvaal and Zimbabwe cratons is located in the CZ as

154 D. Khoza et al. / Precambrian Research 226 (2013) 143– 156

F .7 Ga

t

tT

2rwt2dg

1

mfataKtaafaatr

ig. 10. Schematic illustration of the tectonic evolution of the Limpopo belt from 2o scale). The resistivity image of the LOW profile is projected in the background.

he composite conductive mylonitic feature: the Sunnyside-Palala-shipise-Shear zone system.

The Transpression phase (Fig. 10C): the PTSZ was reactivated at.04 Ga with transcurrent movement at 2.02 Ga (Holzer, 1998) as aesult of the eastward movement of the Zimbabwe craton (coevallyith Magondi belt) relative to Kaapvaal craton. Crustal exhuma-

ion and uplift, a result of Bushveld age magmatic underplating ca..03–1.95 Ga, of the CZ was followed by erosion and the subsequenteposition of the Soutpansberg basin at 1.9 Ga in an extensionalraben-like setting (Kamber et al., 1995a,b; Schaller et al., 1999).

0. Conclusions

The Limpopo belt has been given many geological tags (i.e.,obile or orogenic belt, complex, terrane), in essence to separate it

rom the Kaapvaal and Zimbabwe cratons. From the discussionsbove, the Limpopo belt is perhaps best viewed as a plate tec-onic manifestation of polytectonic structural and metamorphicctivities that resulted from the horizontal collision between theaapvaal and Zimbabwe cratons. Geophysical studies presented in

his work, supported strongly by metamorphic results, appeal ton evolutionary path involving the collision between the Kaapvaalnd Zimbabwe cratons, with the Palala Shear Zone representing a

undamental suture. To this end the Zimbabwe craton representn overriding plate margin with the NMZ being the active marginnd the CZ the leading shelf. Thus, evolutionary models proposinghe separation of the Limpopo belt into separate terranes are notequired. Questions still remain however regarding the timing of

derived from the combination of the MT results and the metamorphic studies (not

the metamorphic events (particularly in the SMZ), orientation andrate of plate of movement in the Archaean. However the presentedmodel, based on all available data including the new MT data, sug-gests that horizontal collisional movement is the most plausible ofall the models that have been presented thus far.

Acknowledgements

The SAMTEX consortium members (Dublin Institute forAdvanced Studies, Woods Hole Oceanographic Institution, Councilfor Geoscience (South Africa), De Beers Group Services, The Univer-sity of theWitwatersrand, Geological Survey of Namibia, GeologicalSurvey of Botswana, Rio Tinto Mining and Exploration, BHP Billiton,Council for Scientific and Industrial Research (South Africa) and ABBSweden) are thanked for their funding and logistical support duringthe four phases of data acquisition. Other members of the SAMTEXteam include: L. Collins, C. Hogg, C. Horan, G. Wallace, M. Mienso-pust (DIAS), A.D. Chave (WHOI), J. Cole, P. Cole, R. Stettler (CGS),T. Ngwisanyi, G. Tshoso (GSB), D. Hutchins, T. Katjiuongua (GSN),E. Cunion, A. Mountford, T. Aravanis (RTME), W. Pettit (BHPB), H.Jelsma (De Beers), P.-E. Share (CSIR), and J. Wasborg (ABB). Thiswork is also supported by research grants from the National ScienceFoundation (EAR-0309584 and EAR-0455242 through the Conti-

nental Dynamics Programme to R.L. Evans), the Department ofScience and Technology, South Africa, and Science Foundation ofIreland (grant 05/RFP/GEO001 to A.G. Jones). The magnetic datacourtesy of the Council for Geoscience South Africa. The Irish Cen-tre for High Performance Computing (ICHEC) is thanked for availing

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he STOKES cluster to carry out the numerical computations. Garygbert and Weerechaii Siripunvarapon are thanked for providing,espectively, the ModEM and WSINV3DMT codes and NaserMeqbelnd Jan Vozar for installing the code on our clusters.

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